The overall aim of this research is an understanding of the forces and interactions that determine and stabilize the structure of the nucleosome and the higher order structure of chromatin. Since reversible modification in these structures must accompany such nuclear processes as chromatin replication and transcription, an understanding of chromatin stability is essential to a complete comprehension of how these processes occur and are controlled in vivo. The problems will be approached through the use of well defined model systems. Nucleosomes and oligonucleosomes will be constructed by reconstitution with histone octamers on cloned DNA fragments containing single or multiple positioning sequences. Either intact chicken erythrocyte histones or histones from which N-terminal tails have been removed by controlled proteolysis will be employed. New methods allow reconstitution with either modified H2A/H2B or H3/H4. With such systems, the following questions will be investigated: (1) What histones, or portions of histones, recognize the positioning signals on DNA sequences? (2) What are the thermodynamic parameters for the dissociation of a defined nucleosome at moderate salt concentrations? What are the contributions of particular histones and portions of histones to nucleosome stability? (3) What are the precise requirements for the extended chain to solenoid transition in chromatin? These experiments will utilize phased, homogeneous oligonucleosomes constructed as above. (4) What roles do the different classes of histone tails play in stabilizing the solenoidal structure?